Integrated circuits (ICs) are usually formed on semiconductor wafers. The wafers are separated into individual dies or chips, and the individual chips are then handled and packaged. The packaging process is one of the most critical steps in the IC fabrication process, both from the point of view of cost and of reliability. For example, the packaging cost can easily exceed the cost of the IC chip itself. In addition, many of the device failures that occur are generally packaging related.
The IC chip should be packaged in a suitable medium that will protect it in subsequent manufacturing steps, as well as from the environment of its intended application. Wire-bonding and encapsulation are the two main steps employed in the packaging process. Wire-bonding connects the leads from the IC chip to terminals on one side of the package substrate. Following wire-bonding of the IC chip to the package substrate, encapsulation is employed to seal the surfaces from moisture and contamination and to protect the wire-bonding and other components from corrosion and mechanical shock.
The terminals to which the leads of the IC chip are wire-bonded are electrically coupled through the package substrate, using vias extending through the package substrate, to package bondpads on a second, opposing side of the package substrate. These bondpads on the package substrate allow the completed package to be connected to other components. Specifically, the exposed bondpads have a solder ball attached to them. As a result, an array of solder balls is formed on the bondpads so that the package may be electrically and mechanically coupled to other circuitry, generally through a printed circuit board (PCB), using the array of solder balls that is referred to as a ball grid array (BGA). When such a BGA is employed to connect the package, it may be referred to as a BGA package.
Unfortunately, the material typically used for the bondpads, for example, copper, oxidizes when remaining exposed throughout the assembly process, until the bondpads are soldered. Oxidation on the surface of the bondpads, as well as contaminants that may gather on the bondpad surface when exposed during the assembly process, often detrimentally affect the metallurgical bond between the bondpads and the corresponding traces on the PCB. Specifically, the dielectric nature of the oxidation may affect the electrical connection between the two, while in some cases also affecting the strength of the bond used to hold the IC package to the PCB. Problems with the electrical connections between the bondpads on the packages and PCBs, as well as the strength of the mechanical bond between the two, can often affect the overall operation and longevity of the assembly.